This invention relates to a differential output circuit. An embodiment of the invention will be described relative to an output circuit for driving a brushless DC motor, and especially to a full wave drive output circuit.
A drive circuit for a brushless motor may be divided into half-wave drive circuits and full wave drive circuits. The half-wave drive causes a unidirectional DC current to flow in each armature winding to generate torque. The full-wave drive circuit causes a DC current to flow in the normal and reverse directions in each armature winding so as to thereby generate torque.
Conventionally, in the full-wave drive circuit case, each armature winding is provided with a common terminal so that positive and negative power sources, with respect to the common terminal, respectively feed currents in the normal and reverse directions to the armature winding.
FIG. 1 shows a conventional example of a two-phase full-wave drive circuit, in which armature windings 9 and 15 are grounded at the common terminals and supply voltages +V.sub.cc and -V.sub.cc are supplied to the circuit.
In FIG. 1, armature windings 9 and 15 are positioned in a relationship of being mutually shifted by an electrical angle of 90.degree.. With respect to the respective windings, Hall generators 3 and 11 are disposed for detecting the rotor position. The Hall generators 3 and 11, as shown in FIG. 2, generate output voltage 16 and 17 which are shifted in phase by 90.degree. and following the rotation of the rotor. The outputs are amplified by amplifiers 6 and 12 so as to be proportional to voltages at both ends of armature windings 9, 15 by means of a feedback circuit. Hence, voltage wave forms at terminals 8, 14 of armature windings 9, 15 with respect to ground terminals become similar to wave forms of the output voltages 16, 17. Other than the phase relation, both two phases act similarly, whereby the following description will concern single phase.
When terminal 8 of armature winding 9 is positive, transistor Q.sub.101 is active and Q.sub.102 is cut-off, and when negative, transistor Q.sub.102 is active and Q.sub.101 is cut-off to thereby respectively cause the flow of normal and reverse currents. In other words, the ground terminals act as the neutral point with respect to the positive and negative power sources. In this instance, only one power source is used once notwithstanding the fact that both positive and negative power sources are required with respect to the neutral point. Hence, there is a defect of reducing the voltage utility percentage of the single power supply to half.